Calcifying cyanobacteria—the potential of biomineralization for carbon capture and storage

Employment of cyanobacteria in biomineralization of carbon dioxide by calcium carbonate precipitation offers novel and self-sustaining strategies for point-source carbon capture and sequestration. Although details of this process remain to be elucidated, a carbon-concentrating mechanism, and chemical reactions in exopolysaccharide or proteinaceous surface layers are assumed to be of crucial importance. Cyanobacteria can utilize solar energy through photosynthesis to convert carbon dioxide to recalcitrant calcium carbonate. Calcium can be derived from sources such as gypsum or industrial brine. A better understanding of the biochemical and genetic mechanisms that carry out and regulate cynaobacterial biomineralization should put us in a position where we can further optimize these steps by exploiting the powerful techniques of genetic engineering, directed evolution, and biomimetics.

Introduction

Strategies to reduce emissions of carbon dioxide (CO₂) from fossil fuels, and hence mitigate climate change, include energy savings, development of renewable biofuels, and carbon capture and storage (CCS). For CCS, several scenarios are being considered. One approach is capture of point-source CO₂ from power plants or other industrial sources and subsequent injection of the concentrated CO₂ underground or into the ocean [1]. An alternative to this point-source CCS method is expansion of biological carbon sequestration of atmospheric CO₂ by measures such as reforestation, changes in land use practices, increased carbon allocation to underground biomass, production of biochar, and enhanced biomineralization [2]. In addition to geological or oceanic CO₂ injection, novel models for point-source CCS based on accelerated weathering and biomineralization are emerging, utilizing either abiotic [3, 4, 5] or biotic [4, 6, 7] processes.

Biomineralization of CO₂ by calcium carbonate (CaCO₃) precipitation is a common phenomenon in marine, freshwater, and terrestrial ecosystems and is a fundamental process in the global carbon cycle [8].

Precipitation of CaCO₃ can proceed by either or both the following reactions:

Ca²⁺ + 2HCO₃⁻ ⇆ CaCO₃ + CO₂ + H₂O

Ca²⁺ + CO₃²⁻ ⇆ CaCO₃

with reaction (2) being the principal path, at least in seawater [9, 10].

Bicarbonate (HCO₃⁻) is ubiquitous in water and is formed via dissolution of gaseous CO₂:

CO_2(aq) + H₂O ⇆ H₂CO₃

H₂CO₃ ⇆ HCO₃⁻ + H⁺

The concentration of carbonic acid (H₂CO₃) is small so the dissolved CO₂ from reactions (3) and (4) occurs predominantly as HCO₃⁻.

A fraction of HCO₃⁻ dissociates to form carbonate (CO₃⁻):

HCO₃⁻ ⇆ H⁺ + CO₃²⁻

The lion's share of global calcification takes place through biotic processes in the oceans. Although the oceans are supersaturated with Ca²⁺ and CO₃⁻, spontaneous precipitation of CaCO₃ in the absence of calcifying (micro)organisms is rare owing to various kinetic barriers [11]. The contribution of microorganisms, particularly cyanobacteria, in CaCO₃ precipitation and sedimentation is substantial and it has played a major role in geological formations since the Archaean Era [12]. Although studies of microbially mediated biomineralization through CaCO₃ precipitation have a long history, the mechanistic details of the different steps are only poorly understood [13^•]. In this review we discuss the potential for microorganisms, specifically cyanobacteria, in calcification, that is conversion of CO₂ to recalcitrant calcium CaCO₃.

We begin our discussion on cyanobacterial calcification and its potential in CCS by a brief description of the general features of cyanobacteria where we elaborate on the carbon-concentrating mechanism (CCM) that allows cyanobacteria to actively take up inorganic carbon (C_i) from the external medium and perform efficient photosynthesis in aqueous environments. We then give an account on microbial biomineralization, specifically as it occurs in cyanobacteria. In this context we return to the CCM and point out the intimate association between CCM and the calcification process. Finally, we ask how biomineralization by calcifying cyanobacteria can contribute to CCS, and we point out research areas that should be prioritized to tackle some of the challenges ahead.